The present invention relates to nut plate fasteners and to repair assemblies for same.
Fasteners are used in the aerospace industry for securing at least two workpieces together. Fasteners used in such installations may include a nut plate and a nut which are part of an assembly. There are many different design configurations of nut plates being used today. Two major classes are riveted nut plates and rivetless nut plates.
In riveted nut plates, two rivets are employed for attaching the body of the nut plate to the workpiece. To eliminate the potential for leakage through the openings of the riveted nut plates, sealant is used between the workpiece and the nut plate.
With regard to rivetless nut plates, some designs provide that a sleeve is flared against a workpiece. One example of this type of rivetless nut plate is disclosed in U.S. Pat. No. 4,732,518, which is hereby incorporated herein by reference in its entirety. The '518 patent illustrates the insertion of a sleeve inside a workpiece against heavy interference forces and then deformation of the sleeve to produce flaring of the end of the sleeve. The sleeve has a serration/lobe configuration thereon with the serration/lobe configuration being long and tapered such that the serrations/lobes extend into the walls of the workpiece. The tapered feature, length and specific geometry are necessary to make installation possible with the method of installation which was chosen for its application. The main object of the '518 patent with its tapered and extended serration/lobe configuration was to enhance the fatigue life of the workpiece by distributing the load throughout the workpiece and providing expansion due to the insertion of the sleeve into the workpiece, and to cold work the material adjacent the perimeter of the workpiece aperture.
Other nut plate designs do not rely on flaring of the sleeve. U.S. Pat. Nos. 5,096,349, 5,245,743, 5,405,228 and pending U.S. patent application Ser. No. 10/272,721 (filed Oct. 17, 2002) and Ser. No. 10/929,701 (filed Aug. 30, 2004) disclose rivetless nut plate designs which do not rely on flaring of the sleeve, and these five items are hereby incorporated herein by reference in their entirety. While some rivetless nut plate designs rely on adhesive for attaching the nut plate to the structure, the designs disclosed in the five items cited above rely on heavily cold-worked holes and high interference engagement utilizing a hardened pin as the installation tool to expand a sleeve element into engagement with a workpiece structure. Because of high level expansion, the friction forces created are intended to retain the nut plate and provide expected mechanical properties.
Once installed, it is sometimes necessary to remove and replace the fastener. Most rivetless nut plates are removed by drilling out the flared sleeve portion, or by drilling out the rivets while holding the nut plate. Drilling out the rivets and the sleeve from the holes requires a special skill.
The process of removing the fastener may affect the size and quality of the hole thereby causing the installation of a new fastener to be impractical. The customary approach to this problem is to design an oversized fastener and to install this oversized fastener in a hole larger than that required for the original fastener.
The design and development of oversized fasteners, however, is expensive. The limited usage, for repair applications only, drives the fastener's unit cost high. There is also the requirement of maintaining these oversized fasteners in inventory and the oversized fasteners are not readily available on the market.
It is noted that if the damage to the hole caused by removing the original fastener is minor, it is possible that the same standard rivetless nut plate may be used such that it will still deliver the expected mechanical properties. Guidelines for this type of repair should be based on test results, empirical analysis, and proper engineering judgment.
The assignee of this invention has also invented a reconditioning tool that allows working and conditioning of the hole from which the original fastener was removed, back to its original condition, such that the same standard rivetless nut plate may again be used. The structure and use of this reconditioning tool is described and illustrated in co-pending U.S. patent application Ser. No. 11/218,076 which was filed on Sep. 1, 2005, which is incorporated herein by reference in its entirety.
There are instances, though, where the damage to the hole cannot be sufficiently reworked/reconditioned by the reconditioning tool described and illustrated in U.S. patent application Ser. No. 11/218,076 such that the same standard rivetless nut plate may not again be used. Where the aperture is either oversized by drilling or cannot be sufficiently reworked/reconditioned, an oversized fastener must currently be used. As it is undesirable to use oversized fasteners, there is a desire that in repair and replacement of the fasteners, that the same size and type of fastener can be used, rather than having to use a specially designed oversized fastener for this purpose. The present invention provides for same.